The present invention is directed to stretch wrap films and methods for their use. In particular, the present invention is directed to stretch wrap films having superior puncture resistance, high elongation to break, high force to stretch the film, high cling force, and overall strength.
The use of thermoplastic stretch wrap films for the overwrap packaging of goods, and in particular, the utilizing of palleted loads is a commercially significant application of polymer film, including generically, polyethylene. Overwrapping a plurality of articles to provide a utilized load can be achieved by a variety of techniques. In one procedure, the load to be wrapped is positioned upon a platform, or turntable, which is made to rotate and in so doing, to take up stretch wrap film supplied from a continuous roll. Braking tension is applied to the film roll so that the film is continuously subjected to a stretching, or tensioning, force as it wraps around the rotating load in overlapping layers. Generally, the stretch wrap film is supplied from a vertically arranged roll positioned adjacent to the rotating pallet load. Rotational speeds of from about 5 to 50 revolutions per minute are common. At the completion of the overwrap operation, the turntable is completely stopped and the film is cut and attached to an underlying layers of film employing tack sealing, adhesive tape, spray adhesives, etc. Depending upon the width of the stretch wrap roll, the load being overwrapped can be shrouded in the film while the vertically arranged film roll remains in a fixed position. Alternatively, the film roll, for example, in the case of relatively narrow film widths and relatively wide pallet loads, can be made to move in a vertical direction as the load is being overwrapped whereby a spiral wrapping effect is achieved on the packaged goods.
Another wrapping method finding acceptance in industry today is that of hand wrapping. In this method, the film is again arranged on a roll, however, it is hand held by the operator who walks around the goods to be wrapped, applying the film to the goods. The roll of film so used may be installed on a hand-held wrapping tool for ease of use by the operator.
Some of the properties desired of a good stretch wrap film are as follows: good cling or cohesion properties, high puncture resistance, high tear resistance in the transverse direction, good machine direction tear resistance, good transparency, low haze, low stress relaxation with time, high resistance to transverse tear when under machine direction tension, producible in thin gauges, low specific gravity and thus high yield in area per pound, good tensile toughness, high machine direction ultimate tensile strength, high machine direction ultimate elongation, and low modulus of elasticity.
Physical properties which are particularly significant for the successful use of thermoplastic films in stretch wrap applications include their puncture resistance, their elongation characteristics, their toughness and their resistance to tearing while under tension. In general, tensile toughness is measured as an area under a stress-strain curve developed for a thermoplastic film and it may be considered as the tensile energy absorbed, expressed in units of ft. lbs./cu.in. to elongate a film to break under tensile load. In turn, this toughness characteristic is a function of the capacity of such films to elongate. The process of stretching the film decreases that capacity. Accordingly, the stretch wrap process will decrease the toughness of the film while it is in its stretched condition as an overwrap as compared to its unstretched form. Generally this low of toughness is proportional to the amount of stretch imparted to the film as it is overwrapping a load of goods.
Currently, different grades of stretch wrap films are commonly marketed for different end uses according to overall film properties. For example, certain stretch wrap films having superior properties for load retention are characterized by requiring a higher force to stretch the film. However, such load retention films generally have only fair or poor puncture characteristics at such stretch conditions. On the other hand, certain stretch wrap films having superior puncture resistance properties have low load retention properties, thus limiting their use.
A need exists to develop superior stretch wrap films characterized by having both superior load retention characteristics and puncture resistance characteristics. Such films could be used in a wider variety of end applications and thus not unduly limit users of stretch wrap films to selectively choosing a film based on its properties prior to initiating a stretch wrap application.
The present invention sets forth multilayer stretch wrap thermoplastic films that are characterized by having improved cling, tear, elongation, and puncture resistance properties. The stretch wrap films can be used in cling/cling film applications. The stretch wrap film is advantageously employed in wrapping articles while under tension whereby the film itself is stretched generally to at least 2000% of its original length.
The stretch wrap film of the present invention can be prepared as a multilayer film construction. In such a construction the two outside layers have a cling force to each other of at least about 140 grams/inch. At least one of the outside cling layers preferably comprises at least 40 weight percent of a puncture resistant polyethylene resin having a polydispersity (Mw/Mn or MWD) of from 1 to 4, a melt index of from 0.5 to 10 g/10 min., and a melt flow ratio (I20/I2) of from 12 to 22. This puncture resistant polyethylene resin is preferably produced utilizing metallocene catalyst polymerization techniques. The multilayer film also contains an interior film layer, located between the outside cling layers, that provides load retention and transverse direction tear resistance. The resin comprising the tear resistant layer forms a film having a higher transverse direction tear resistance than films constructed with the resins that comprise the outside cling layers.
The multilayer films of the present invention have been found to display unexpectedly superior films properties compared to other film constructions, surprisingly without undue degradation of other important film properties. The incorporation of the puncture resistant resin in at least one outside cling layer with an internal transverse direction tear resistant layer, using the resins for those layers as set forth herein, has produced an overall film that has superior elongation characteristics. Also, the films of the present invention display enhanced transverse direction tear properties without a concomitant loss of machine direction tear properties.